Subchronic and chronic studies with Firemaster FF-1 (mixture of polybrominated biphenyls) were conducted in both rats and mice of both sexes. The animals were given 125 oral doses of Firemaster FF-1 over a 6-month period at 0.1, 0.3, 1.0, 3.0 and 10.0 mg/kg/day (5 da/wk). The animals were sacrificed at the end of six-month treatment and the following parameters were studied: food consumption, body weight gain, hematology, clinical pathology, anatomic pathology and tissue porphyrins. Animals from each dose group are being held for life-time observations. The purpose of these toxicological studies is to determine the subchronic and chronic effects from exposure to Firemaster FF-1 in the rat and mouse.